Signal generator or modulator



Jan. 18,1949. J. R. SHONNARD SIGNAL GENERATOR OR MODULATOR 2 Sheets-Sheet 1 Filed Aug. 15, 1946 INVENTOR. JR./2012ndrd -ATTOR/VEY Jan. 18, 1949. J. R. SHONNARD 2,459,293

' v S IGNAL GENERATOR OR MODULATOR Filed Aug. 15, 1846 2 .Shaets-Shqet 2 AMP.

STEADY LIGHT NOD. LIGHT S'I'EADY LIGHT M00. LIGHT uvmvrm J R. Ska/lizard AT 7010/5 Y Patented Jan. 18, 1949 SIGNAL GENERATOR QR MODULATOR John lit. Shonnard', New York, N. Y., assignor to Times Facsimile Corporation, New York, N. Y., a corporation of New York Application August 15, 1M6, Serial No. 690,822

This invention relates to methods and apparatus for generating electric signals, and more particularly to a photocell modulator for telefacsimile systems, picture transmission and the like.

In general terms, the object of the invention is to provide an improved photocell modulator or modulating method for generating a modulated carrier free of any direct-current component.

Another object of the invention is to provide an improved picture scanning arrangement for facsimile systems and the like having decreased signal distortion and other advantages.

In the case of ordinary photocell modulators used in facsimile systems, the output current is a modulated carrier which is amplified before being impressed upon the line circuit by a multistage amplifier.

The photocell in the modulator comprises a photo-electric cathode and a spaced anode. When an external battery is connected to the cell, the magnitude of the current traversing the cell at any instant is dependent upon the instantaneous intensity of illumination of the cathode.

Thus the output of the cell is a rectified current, varying in magnitude as a function of the light variations impressed thereon. Since for the steps of amplification and transmission it is desirable to convert these variable direct-current signals into amplitude variations of an alternating current or carrier frequency, a light-chopper has been employed to interrupt the illumination of the cell periodically and thus superimpose a constant-frequency variation upon the directcurrent signals. In an alternative arrangement,

the cell variations have been utilized to unbalance a bridge upon which a carrier potential is in pressed, and thus generate a variable-amplitude carrier controlled by the cell. In both types of modulators, prior to the instant invention, the direct-current component or bias potential of the signals is present in the output current of the modulator and is of greatermagnitude than the modulated carrier. This DC component or potential produces distortion of the signals in the amplifier. It cannot be completely filtered out although some filtering is usually employed with consequent distortion of the lowerv frequency components and reduction of the gain or ampli fication factor of each stage of the amplifier. In

transmission, the direct-current component. of the signals also produces-further distortion-which 3 Claims. (01. 250-415) V 2 has been unavoidable in the systems in general use prior to the present invention. v In the preferred scanning apparatus embody.- ing the present invention, a photocell modulator is employed having a plurality of cathodes so connected to the source of carrier current and the output circuit as to produce a modulated carrier in the output circuit which is free from the direct-current bias normally produced in the usual photocell modulator. The preferred form of the invention comprises a photocell having two oppositely connected cathodes which balance out the direct-current component ofthe signal resulting from the light excitation of the photocell, the cathodes being alternately operative'on'alternate half-cycles of the impressed carrier so that a modulated carrier current free from bias traverses the output circuit, said carrier current being modulated in amplitude in accordance with the instantaneous light excitation of the cell.

Accordingly, it is anotherobject of the invention to provide a scanning arrangement for picture transmission capable of generating a light-modulated carrier which can be amplified and transmitted without appreciable distortion. In this manner an efficient high-gain amplifier can be employed without producing objectionable distortion of the picture signals.

Other objects and advantages of the invention will appear from the following description of the preferred embodiments thereof shown in the accompanying drawings, wherein Fig. 1 is a circuit diagram of a'photocell modu lator as applied to a telefacsimile transmitter;

Fig. 2 is a perspective view illustrating the construction of the photocell shown in Fig. 1;

Fig. 3 is a detail view illustrating the use of an optical wedge in conjunction with the photocell shown in Figs. 1 and 2; v

Fig. 4 is a circuit diagram of modification; .Eig. 5 is an explanatory diagram corresponding tothe circuit diagram of Fig. l or Fig. 4;

Figs. 6 and 7 are graphs of the signal currents showing the eii'ect of the direct current bias on the wave-shape of the signals; and Fig. 8. is a cross-sectiona1 view of a modified photocell that may be employed in carrying out the invention.

Referring to Fig. 1 of the drawings, the elements of a telefacsimile transmitter are shown comprising the oonventional"rotating drum ill carrying the picture or copy to be transmitted; an exciter'lamp'il and condenser lens 52 .for. illuminating the'copy being scanned by the transboth half cycles of the applied electromotive force as indicated by the curve 43 in Fig. '7, by interconnecting the cathodes and anodes of the photo-cell so that each pair of electrodes is operative on each succeeding half-cycle of current. The resulting output current where the bridge network is employed is represented by the curve 44 in Fig. 7. It will be apparent that the signal wave produced is a modulated alternating current without any direct-current component. Such a signal can be amplified in a conventional vacuum-tube amplifier without distortion. In actual practice it is found that the distortion in transmission is reduced and improved recording can be efi'ected when the described modulating method is employed.

In order to avoid the loss of light projected through the space between the two cathodes of the #920 photocell, a split prism or optical wedge 46 can be arranged in front of the cell as shown in Fig. 3, or with two separate cells of the singlecathode type. The wedge 46 consists of two prisms with their edges abutting at the center of the beam of light reflected from the copy as shown, whereby half the light is directed to the active surface of each cathode. Other equivalent optical elements may be employed in a similar manner instead of the wedge 46. Another preferred form of photocell modulator embodying the invention is shown in Fig. 4. In this modification, the balanced bridge and amplifier are connected to the photocell 48 in the same manner as shown in Fig. 1 except that the photocell is provided with a third anode 49 disposed behind the cathodes of the cell and the condenser 35 is omitted. The capacity between the electrode 49 and the cathodes of the photocell balances the capacity between the other electrodes 20 and the cathodes, and thus no external condenser 35 is required. The advantage of this construction is that the elements of the capacity represented by the electrode 49 and the cathodes l9 are sealed inside of the photocell envelope and are thus not afl'ected by moisture and the like. However, the anode must be accurately positioned so that the proper interelectrode capacity is obtained. Since the operation of the modulator shown in this figure is the same as that shown in Fig. 1, no further explanation thereof is deemed necessary.

In some applications the anodes 20 are unnecessary since each cathode could function as an anode during the half-cycle when the other cathode is at a negative potential. A photocell of this type is illustrated in Fig. 8. As shown, the photocell 5| comprises two photo-electric cathodes 52 and 53, arranged substantially perpendicular to each other and overlapping so that no light from the source or scanning spot 54 escapes between the cathodes. A two-part balancing electrode insulated to prevent space current and consisting of wires or rods 55 and 55, one mounted behind each of the respective cathodes, is provided for the purpose of obtaining a capacity balance when the cell is used with a bridge network of the type shown in Figs. 1 and 4. The total capacity between the electrode 55, 56 and cathodes 52 and 53, is equal to the capacity between the two cathodes. Preferably the parts of the electrode 55, 56 which are adjacent the cathodes or exposed to electron emission therefrom, are covered or coated with insulation as indicated, to obviate the flow of electron current to this electrode. The cathodes 52 and 53 may be connected, for example, in the same manner as the cathodes of the photocell 48.

While several embodiments of, the invention have been described by way of example, other modifications will occur to those skilled in the art and may be made without departing from the scope of the invention. For example, in certain applications separate photocells may be employed instead of a single photocell provided with a plurality of photo-electric cathodes, although the latter modification is usually preferable.

I claim:

1. In a photocell modulator, in combination, a photocell provided with electrodes including two photo-electric cathodes and an anode, an output circuit for said photocell. said circuit including one of said cathodes, a source of alternating electromotive force and means comprising a balancing network connected between said source and said output circuit for generating a modulated carrier in said output circuit upon equal light modulation of both of said cathodes, said network including the space paths between electrodes of said photocell and also including external impedances connected to said source and the electrodes of said photocell.

2. In a photocell modulator, in combination, a photocell provided with two separate photo-electric cathodes and a balancing electrode in spaced relation to said cathodes, a balanced bridge network including said photocell in one arm thereof, a carrier-current source connected to opposite terminals of the bridge network, an output circuit associated with said photocell and bridge network, means to illuminate the cathodes of the cell equally with light varying in a predetermined manner and connections for said cathodes to render each operative to establish current flow in an area of the bridge on alternate half-cycles of said carrier-current source.

3. In a photocell modulator, in combination, a photocell provided with two separate photoelectric cathodes and a balancing electrode in spaced relation to said cathodes, a source of carrier current, a balancing network connected to said source and comprising the space paths between said electrodes of the photocell, means including said source for rendering said cathodes alternately operative to establish current flow to the other cathode as an anode at a relatively positive potential on alternate half-cycles of said carrier-current source and means to apply li h excitation equally to said cathodes.

JOHN R. SHONNARD.

REFERENGES crrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,837,364 Ives Dec. 22, 1931 2,050,737 Schriever Aug. 11, 1936 2,140,373 Vedder Dec. 13, 1938 2,287,965 Borberg June 30, 1942 

